Techniques have been developed to attempt to overcome this difficulty, notably for radars on board a mobile platform and in the case where the positions of the detections of unwanted spikes are at least partly decorrelated over several successive antenna revolutions. These techniques consist in testing the stability of presumed detections in the scanning space. This involves determining pre-detection states over N successive antenna revolutions, then in shifting the positions of the pre-detections in a unique frame of reference for the N antenna revolutions (so that an immobile target is located at the same position in this unique frame of reference for the N antenna revolutions), and finally in testing, in this unique frame of reference, the confirmation of each pre-detection over the N antenna revolutions. One difficulty lies in the fact that, since the radar on principle performs its position measurements using azimuth and distance, the state of the pre-detections over one antenna revolution is obtained in a polar representation. At the next antenna revolution, the position of the radar carrier having been translated, it is not possible to directly superpose the pre-detections obtained in the polar frames of reference of the successive antenna revolutions. Another difficulty lies in the fact that the potential targets are mobile and that, even if the difficulty related to carrier movement is circumvented, the positions of the targets over several antenna revolutions cannot be directly superposed. Again, this is a problem that the present invention proposes to solve.
To attempt to overcome these difficulties, a known solution consists in integrating the pre-detections in Cartesian coordinates (X,Y) revolution by revolution. For each antenna revolution, the positions of the radar pre-detections, initially obtained in polar coordinates, are transferred into a planar Cartesian frame of reference whose origin is close to the position of the carrier. The shifts of the pre-detections obtained over the successive antenna revolutions are simply produced by a shift corresponding to the movement of the carrier in (X,Y) between the starts of the successive antenna revolutions. After shifting the pre-detections of the N last revolutions in the frame of reference (X,Y) of the last revolution, a detection test of K/N type is performed in each cell of the plane: this involves testing whether the number of pre-detections is greater than a number K over N antenna revolutions. A major drawback of this solution is that the elementary integration cell on the radar map is a square or a rectangle, and that the ratio between this cell and an elementary radar resolution cell is variable. In fact, an elementary radar resolution cell is defined by the aperture of the antenna lobe in azimuth and by the radar resolution in distance. The radar resolution is therefore notably variable as a function of distance. Therefore, detection performance is not uniform.
To attempt to overcome this drawback, another solution is based on the creation of N maps of pre-detections in polar coordinates (ρ,θ) with increments of azimuth θ and of distance ρ which are compatible with possible movements of a target over the N revolutions used for detection. The pre-detections over the N maps are shifted in the reference frame of the plane corresponding to the current antenna revolution. A test is then carried out to verify whether, in each cell, the number of pre-detections is greater than a number K over N antenna revolutions. A major drawback of this solution is the large memory space necessary to store the maps of pre-detections over the N revolutions, as well as the computing power needed to shift the pre-detections obtained in a reference frame (ρ,θ) centered on the origin of each of the N revolutions in the reference frame of the current revolution. Another drawback of this solution is that, the size of the integration cell being large in relation to the elementary distance resolution of the radar, the movement of the target over the duration of the N revolutions often being large in relation to the dimensions of the target, and the only test applied to the pre-detections being a K/N test, the entry threshold for establishing the pre-detections must be high enough to limit false alarms at the processor output. This leads to a loss of sensitivity when detecting small targets in the radar clutter.
Patent application GB 2435 138 A describes a device for integrating pre-detections revolution by revolution. It is based on the use of two windows for the shift and test operations: a rectangular window surrounding the radar resolution cell and a window delimiting a distance bracket. This solution is a mixture of the previously described solutions involving integration in Cartesian coordinates and integration in polar coordinates, with which it shares the drawbacks mentioned previously.